The basic objective of this project is to analyze the capacities of primitive mammalian hematopoietic progenitor cells to undergo self- renewal and differentiation in relation to their position within the cell cycle. By employing cell separation techniques, both with centrifugation and a unit gravity, rapidly growing murine bone marrow will be sedimented to obtain fractions enriched in pluripotent (the in vivo splenic colony-forming unit, or CFU) and unipotent (erythropoietin- responsive cells, ERC) progenitor cells. Proliferating CFU in each cell-cycle phase, including Go 'rest', will thus be obtained and their capacity to undergo self-renewal tested by double in vivo spleen colony transplant experiments. The capacity of CFU for differentiation will be tested by the in vitro CFC-agar assay method. Both self-renewal and differentiation capacities will be assessed in relation to the position of CFU within the cell cycle. Unipotent ERC will be likewise synchronized by sedimentation and the capacity of these cells to undergo erythroid differentiation assessed as a function of the cell cycle. These experiments are expected to provide insight into the relationship of progenitor cells to one another and into the nature of the capacities of progenitor cells for differentiation and self-renewal with perspective to the cell cycle.